High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319 and in Pengilly, U.S. Pat. No. 3,047,539. These patents disclose that the polyesters are particularly advantageous as film and fiber-formers.
With the development of molecular weight control, use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection moldable compositions. Poly(1,4-butylene terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Workpieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.
A useful family of such compositions are those which are glass-reinforced, e.g., with from about 20 to about 40% of filamentous glass, based on the weight of glass and polyester component. Unfortunately, the addition of reinforcing fillers, including glass, mica, talc, and the like, even in very small amounts, always causes the compositions to become brittle, i.e., to lose the inherent toughness of the polyester component.
For example, addition of between 1 to 4 percent by weight of glass filaments, calcium silicate, talc and mica reduces the elongation at break and ductile (Gardner, falling dart) impact strength of unreinforced poly(1,4-butylene terephthalate), even though the strength, modulus and heat deflection temperatures are increased.
A unique reinforcement for high molecular weight linear polyesters, namely potassium titanate in the form of single crystal filaments, has now been discovered. At low levels of concentration of this reinforcement, the polyester resins are found to have elevated strengths, moduli and heat deflection temperatures, while retaining a surprising degree of practical toughness, as measured by elongation and Gardner ductile impact strength.